Nitinol is a nickel-titanium alloy and probably the best-known representative of the shape-memory alloys. Nitinol has a cubic crystal structure which comprises approximately 55 wt. % nickel and the remainder titanium. The alloy is usable up to 650° C., is corrosion resistant, and is very strong. The alloy is pseudo-elastically deformable up to approximately 8%.
The requirement for the shape-memory effect is a so-called martensitic conversion in which the participating phases, high-temperature phase (austenite) and low-temperature phase (martensite), have ordered lattice structures. Upon the conversion into shape-memory alloys, only very slight elastic tensions occur. The irreversible plastic deformation caused by dislocation movement is nearly completely avoided by the formation of so-called twins in specially oriented martensite plates. The body-centered cubic austenite converts upon cooling into a twinned martensite structure. This conversion occurs without diffusion, with release of heat, by folding procedures, and is not connected to a change of the shape of the object. The martensite may be easily deformed by detwinning, and this deformation is permanent as long as the material remains below the conversion temperature. However, if the deformed martensite is heated, the original crystal orientation of the high-temperature phase, and thus the original shape, is resumed upon exceeding the conversion temperature. The conversion austenite/martensite and the reconversion martensite/austenite occur at different temperatures.
The two phases display characteristic differences in the strength behavior. The solidification behavior of the martensite is quite unusual. It is characterized by the so-called martensite plateau, a range having very low solidification. The deformation by detwinning occurs here. When this deformation capability is exhausted (after approximately 8% elongation), other types of deformation must be activated. A second elastic range thus adjoins the martensite plateau. Upon reaching the true yield strength, the deformation occurs conventionally by dislocation movement. The deformation in the range of the martensite plateau may be reversed by heating. An unusual tension-elongation diagram is also observed upon the deformation of austenite below a limiting temperature for the tension-induced martensite formation. The effect of the so-called pseudo-elasticity or super elasticity occurs here, which is to be attributed to the formation of tension-induced martensite.
The martensite formation may be caused not only by thermal, but also by mechanical propulsive forces. If a shape-memory alloy is strained in the high-temperature state, martensite twins are induced which immediately detwin. With increasing tension, more martensite is induced and detwinned. The martensite thus arising is thermally unstable, however, i.e., upon abeyance of the external tensions, a reconversion occurs immediately. The sample assumes its original shape again. At temperatures above a specific limiting temperature, martensite may no longer be induced. It is now easier to generate and move dislocations. The tension/elongation diagram of austenite is then similar to that of conventional alloys.
Depending on the preceding thermomechanical treatment, three possible effects may thus be differentiated. In the one-way effect, a large, lasting deformation is reversed by heating by a few degrees Celsius. In the two-way effect, the component made of the shape-memory alloy remembers two previously applied different shapes and assumes these alternately upon heating and cooling. Finally, the pseudo-elasticity causes an elastic behavior, almost constant forces being exerted over large deformation distances.
Shape-memory alloys, in particular, nitinol, are used in medical technology in the form of, inter alia, self-expanding stents. A stent is a medical implant which is introduced into specific organs to support their walls all the way around. The nitinol stent is a small tubular support structure comprising nitinol, which may assume a compressed state having a small diameter and an expanded state having an enlarged diameter predefinable for the intended purpose.
Stents are used, on the one hand, in blood vessels, especially the coronary blood vessels, to prevent a renewed closure after their expansion. Such a treatment is referred to as stent angioplasty or PTCA. On the other hand, stents are used in cancer treatment for keeping open constrictions caused by malignant tumors of, for example, the airways, bile ducts, or the esophagus, after an expansion.
The nitinol stent is fixed on a catheter system in the compressed state by external polymer tubing, the protective envelope, and is thus brought to the implantation location. The stent is now released by the retraction of the external polymer tubing. The stent opens independently and supports the vascular wall/organ wall as a result of the pseudo-elastic behavior of the nickel-titanium alloy.
Self-expanding nitinol stents on the market may generally be divided into two categories in regard to their design, and especially their degree of cross-linking, namely, open-cell and closed-cell designs.
Open-cell designs are very flexibly constructible, but have the disadvantage that they may not be restored in the protective envelope of the catheter without the use of additional aids due to low cross-linking in the axial direction.
Closed-cell designs, in particular, have a support structure which comprises peripheral struts around the circumference and which are linked to one another in the axial direction via connection struts. Because of the high degree of axial cross-linking via the connection struts, closed-cell designs may be retracted again into the protective envelope of the catheter system after partial release (so-called restorability). The high degree of cross-linking by the axial connection struts ensures a significantly higher bending stiffness than the stent designed as open-cell, however. For stents made of nitinol having a closed-cell design, the need exists to increase the axial flexibility of the stent without restricting the restorability.